The present invention relates to controlling the flow of fluids such as gases.
The prior art, as illustrated in FIG. 1, describes controlling fluid in a fluid path with a flow control system having a positive shutoff valve located downstream of a modulator valve. An example of a modulator is proportional control valve available from MKS Instruments, Inc. under the trade designation 248 valve. In this valve, an armature assembly is suspended and centered in the valve body by a top spring and a bottom spring. The bottom spring is dual purpose and provides both centering and closing force while the top spring provides only centering force. Located in such a fashion, the armature may be lifted by varying displacements by a solenoid coil, thereby controlling flow.
In operation of this valve, gas enters the valve and flows into the valve chamber. As the armature is lifted by the solenoid, fluid flows through an orifice assembly, a side drill passage and out of the valve. The valve seat contains an elastomeric plug which bears on the orifice. However, because of the low seating pressure and the permation rate of gases through the elastomeric material, positive shutoff is not possible. Thus, a modulation valve of this type must be utilized in cooperation with a positive shutoff valve to achieve the desired dual purpose of modulation and positive shutoff.
A controller controls both valve and modulator. This system creates a dead volume of unknown pressure and volume between the components that needs to be purged or flushed from the system. To address this situation, the prior art has provided a flow system which produces a gas flow having sharp leading and trailing edges (i.e. no dead volume) by providing extra hardware such as tubes and pumps to the flow control system described above.